LibWeb/DOM: Implement "is closed-shadow-hidden" algorithm

Author: https://github.com/AtkinsSJ
Commit: 3fbd3146a1
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5113

Libraries/LibWeb/DOM/Node.h

@@ -294,6 +294,8 @@ public:
         return const_cast<Node*>(this)->shadow_including_root();
     }
 
+    bool is_closed_shadow_hidden_from(Node const&) const;
+
     bool is_connected() const;
 
     [[nodiscard]] bool is_browsing_context_connected() const;